1. Field of the Invention
The present invention is related to sealed lead acid batteries characterized by an active material in a positive or negative electrode.
2. Description of the Prior Art
In general, a lead acid battery is a battery which has a stable characteristic at a lower cost as a secondary battery and which is used widely as an electric power source of a movable type for use in a portable electronic apparatus, a starter for automobile or a golf cart and a stationary type for use in a back up electric source for computer.
Recently, much demand has been directed to a sealed lead acid battery of high quality for use as an electric power source in an electric car. Specially, an important problem is to realize a high energy density discharge and increase a cycle life at a high efficiency discharge (a high current in a short time).
These problems are greatly related to an active material and an electrolyte (sulfuric acid). As a practical matter, the actual energy density is one third of the theoretical value. The reason is that although it is assumed that the active material and the electrolyte react completely with each other, it is believed that only a small amount of sulfuric acid can contribute to the discharge reaction in an actual battery. Specially, in the sealed lead acid battery, sulfuric acid impregnated in a separator reacts mainly with the surface of the electrode plate and sulfuric acid impregnated previously in the active material reacts with the electrode plate at a close position to a grid for an electric current collector. It is believed that a liquid type lead acid battery also has a lower energy density due to incomplete diffusion of the sulfuric acid into the active material and incomplete reaction with the active material.
It is believed that the capacity decay in the cycle life occurs when the active material grows to a crystalline lead sulphate in a passive state during the repetition of charge and discharge operation and has no effective function on the battery reaction. Another reason is that pores having a size less than 0.1 micron increase while pores having an intermediate size of 0.1 to several microns decrease, since small pores less than 0.1 micron causes sulfuric acid to diffuse hardly into the active material.
Therefore, in order to improve initial capacity, energy density and discharge characteristics efficiently, there is proposed a method to increase the amount of sulfuric acid in order to increase the energy density or porosity of the active material (a decrease in the apparent density) in order to improve the diffusion rate of sulfuric acid. This method, however, has a disadvantage such as the cycle life is shortened because the increase in the pore size causes the generation of crystalline lead sulphate of a larger grain size which results in a more accelerated passive state of the active material.
In order to prevent the passive state of crystalline lead sulphate which is essentially an insulating material, a possible method is to mix the active material with a conductive material such as anisotropic graphite or tin oxide or zeolite having sulfuric acid impregnated therein and porous silica coated with titanium (for example, see Japanese Patent laid-open Publication No. 54-60429, 58-12263, and 3-22355). Another disclosed method is to add porous silica having an elastic property in order to absorb the strain of expansion and shrinkage at the charge and discharge operation so that the cycle life is improved (for example, see U.S. Pat. No. 4, 548,835, U.S. Pat. No. 4, 748,093).
It is possible to prevent the passive state of the lead sulphate in order to improve the cycle life to some extent by adding a conductive material such as tin oxide to the active material. However, sulfuric acid shows no change in the diffusion rate in view of the discharge at a high efficiency. A supply of sulfuric acid from the separator is not sufficiently quick so that a decrease in the capacity can not be prevented during the discharge at a high efficiency. Further, an addition of zeolite to the active material results in an increase in the amount of sulfuric acid at a position near the active material and is effective for an increase in the capacity at a high efficiency. However, alkali metal oxide such as sodium oxide or potassium oxide contained in the zeolite solves into the active material and impairs the active material. As a result, the addition of zeolite has no effect on the cycle life.
On the other hand, the addition of porous silica coated with titanium permits sulfuric acid to be distributed uniformly around the active material to some extent and has an effect to improve the discharge characteristics at a high efficiency and initial capacity. However, this method has a disadvantage that sulfuric acid hardly diffuses into the inside of particles of the active material, thereby lowering the effect on the improvement of the discharge characteristic. Further, the porosity increases during mixing so that the cycle life becomes short. In the case of the elastic porous silica, there is found a little effect on the improvement of the cycle life and there is found no improvement of the energy density because of the small increment of the amount of sulfuric acid and the large specific gravity of the elastic porous silica.